Information upon which to base the rational utilization of propranolol in the treatment of ventricular arrhythmias is not currently available. Our preliminary studies indicate that many patients currently are receiving sub-optimal therapy or are classified as therapeutic failures when treated with propranolol according to present practices. We propose therefore to perform the basic pharmacologic and pharmacokinetic studies necessary for optimal propranolol therapy of ventricular arrhythmias. The antiarrhythmic efficacy of a range of propranolol dosages will be evaluated in patients with stable ventricular arrhythmias. The suppression of ectopic ventricular depolarizations by the drug in comparison with a placebo control will be determined by computer analysis of telemetered ECG data. Determination of the concentration of propranolol that is associated with antiarrhythmic effect will provide a basis for improved individualization of therapy with this drug which has a wide range of plasma concentrations associated with any dose. Side effects of the drug will be evaluated by a blinded observer. Clinical studies performed to date to determine the effects of propranolol on the myocardial conduction system have used doses which we have found inadequate for suppression of ventricular arrhythmias. Therefore, intracardiac electrophysiological studies will be performed in selected patients in order to determine the effects of intravenous propranolol when infused to suppress ventricular arrhythmias. The information provided from this study should provide a more rational basis for the use of propranolol in the treatment of ventricular arrhythmias.